1. Field of the Invention
This invention relates to a novel catalytic cracking process to produce motor fuels. In particular, this invention relates to an improved catalytic cracking process for producing motor fuel involving the use of an additive catalyst such as ZSM-5 in conjunction with a conventional zeolite containing cracking catalyst to thereby increase gasoline octane number and gasoline plus alkylate yield. The additive catalyst utilized in the novel process of this invention is a special class of zeolites well known in the art which have been prepared by in-situ crystallization of a preformed clay aggregate as opposed to prior art routes.
2. Description of the Prior Art
Hydrocarbon conversion processes utilizing crystalline zeolites have been the subject of extensive investigation during recent years, as is obvious from both the patent and scientific literature. Crystalline zeolites have been found to be particularly effective for a wide variety of hydrocarbon conversion processes including the catalytic cracking of a gas oil to produce motor fuels and have been described and claimed in many patents, including U.S. Pat. Nos. 3,140,249; 3,140,251; 3,140,252; 3,140,253; and 3,271,418. It is also known in the prior art to incorporate the crystalline zeolite into a matrix for catalytic cracking and such disclosure appears in one or more of the above-identified U.S. patents.
It is also known that improved results will be obtained with regard to the catalytic cracking of gas oils if a crystalline zeolite having a pore size of less than 7 Angstrom units is included with a crystalline zeolite having a pore size greater than 8 Angstrom units, either with or without a matrix. A disclosure of this type is found in U.S. Pat. No. 3,769,202. Although the incorporation of a crystalline zeolite having a pore size of less than 7 Angstrom units into a catalyst composite comprising a large pore size crystalline zeolite (pore size greater than 8 Angstrom units) has indeed been very effective with respect to raising of octane number; nevertheless, it did so at the expense of the overall yield of gasoline.
Improved results in catalytic cracking with respect to both octane number and overall yield were achieved in U.S. Pat. No. 3,758,403. In said patent, the cracking catalyst was comprised of a large pore size crystalline zeolite (pore size greater than 7 Angstrom units) in admixture with ZSM-5 type zeolite wherein the ratio of ZSM-5 type zeolite to large pore size crystalline zeolite was in the range of 1:10 to 3:1. Effective cracking processes were disclosed as being achieved when the catalyst was used to obtain the inherent advantages realized in moving bed techniques, such as the thermofor catalytic cracking process (TCC) as well as in fluidized cracking processes (FCC).
The use of ZSM-5 type zeolite in conjunction with a zeolite cracking catalyst of the X or Y faujasite variety is described in U.S. Pat. Nos. 3,894,931; 3,894,933; and 3,894,934. The two former patents disclose the use of ZSM-5 type zeolite in amounts up to and about 5 to 10 weight percent; the latter patent discloses the weight ratio of ZSM-5 type zeolite to large pore size crystalline zeolite within the range of 1:10 to 3:1.
The addition of a separate additive catalyst comprising one or more members of the ZSM-5 type has been found to be extremely efficient as an octane and total yeild improver when used in very small amounts in conjunction with a conventional cracking catalyst. Thus, in U.S. Pat. No. 4,309,179, it was found that only 0.1 to 0.5 weight percent of a ZSM-5 type catalyst added to a conventional cracking catalyst under conventional cracking operations could increase octane by about 1 to 3 RON+0 (research octane number without lead).
Recently, improvements have been made with respect to enhancing the hydrothermal stability of zeolites such as ZSM-5 by silver incorporation and such is disclosed and claimed in copending application Ser. No. 587,415 filed Mar. 5, 1984. Another copending application, Ser. No. 588,253, filed Mar. 12, 1984, teaches the effect of crystallite size on hydrothermal stability.
In order to reduce automobile exhaust emissions to meet federal and state pollution requirements, many automobile manufacturers have equipped the exhaust system of their vehicles with catalytic converters. Said converters contain catalysts which are poisoned by tetraethyl lead. Since tetraethyl lead has been widely used to boost the octane number of gasoline, refiners now have to turn to alternate means to improve gasoline octane number.
One method of increasing octane number is to raise the cracker reactor temperature. This method, however, is very limited since many units are now operating at maximum temperatures due to metallurgical limitations. Raising the cracker reactor temperature also results in increased requirements for the gas plant (i.e., gas compressor and separator). Since most gas plants are now operating at maximum capacity, any increased load could not be tolerated by the present equipment.
An alternative method has been to mix an additive catalyst such as ZSM-5 to the cracking catalyst as described above. Generally, the octane gain of a ZSM-5 containing cracking catalyst is associated with gasoline (C.sub.5.sup.30) yield decrease and correspondingly higher yields of C.sub.3 and C.sub.4 gaseous products. As the freshly added ZSM-5 undergoes hydrothermal deactivation the octane enhancement is reduced and additional ZSM-5 must be added to maintain the desired octane level.
As can well be appreciated in the foregoing, it would be extremely desirable to have a more steam stable ZSM-5 additive which would in effect reduce the additive catalyst requirement to maintain a given octane level.
The combined methods for synthesis of zeolites are extensively described in the literature. Generally the aluminosilicate zeolites crystallize from aqueous systems of high pH containing sources of silica, alumina and a source of a suitable cation, typically sodium. The system is maintained under hydrothermal conditions such as room temperature up to 200.degree. C. and higher at autogeneous pressure until the crystalline product is formed. The ratio of silica to alumina is found to be at least 2 (silicon/aluminum=1) and ranges upwardly, depending on the specific zeolite and the conditions of synthesis. The zeolites made available on a commercial scale have pore sizes varying from about 4 to about 10 or higher Angstrom Units (A).
Some newer zeolites have exhibited extremely high silica/alumina ratios. A typical such zeolite is ZSM-5 described in Argauer et al. U.S. Pat. No. 3,702,886. That patent describes crystallization of ZSM-5 by hydrothermal treatment of a reaction mixture containing sources of silica, alumina and an alkali metal oxide plus a quaternary ammonium compound such as a tetrapropylammonium salt. A somewhat similar zeolite is ZSM-11 described in Chu U.S. Pat. No. 3,709,979, where a quaternary ion compound is also employed. U.S. Pat. No. 3,941,871 to Dwyer et al. is concerned with the special case of ZSM-5 in which the alumina content is vanishly small, aptly called "organo-silicate". Other zeolites, similar in structure to ZMS-5, include ZSM-11 (U.S. Pat. No. 3,709,979 to Chu), ZSM-12 (U.S. Pat. No. 3,832,449 to Rosinski et al.), ZSM-23 (U.S. Pat. No. 4,076,842 to Plank et al.), ZSM-35 (U.S. Pat. No. 4,016,245 to Plank et al.), ZSM-38 (U.S. Pat. No. 4,046,859 to Plank et al.) and ZSM-48 (U.S. Pat. No. 4,397,827 to Chu). These zeolites have a silica/alumina ratio greater than 12 and a Constraint Index between 1 and about 12.
Zeolites are often combined with a porous matrix to provide a catalyst composition. The matrix tends to improve the activity and/or selectivity of the catalyst in certain hydrocarbon conversion processes. Inert materials which serve as the porous matrix serve as diluents to control the amount of conversion in a particular process so that products can be obtained economically and in an orderly manner without employing other means for controlling the rate of reaction. The material employed as the porous matrix may be active or inert. The porous matrix also functions as a binder for the zeolite catalyst to provide a composition having a good crush strength. Inorganic materials, especially those of a porous nature are preferred. Of these materials inorganic oxides such as clay, chemically treated clay, alumina, silica, silica-alumina and the like are particularly preferred because of the superior porosity, attrition resistance and stability they provide to the zeolitic composition. The zeolite can be combined, dispersed or otherwise intimately admixed with the porous matrix in such proportions that the resulting product contains from 1 to 95% by weight, and preferably from 1 to 70% by weight, of the zeolite in the final composite. For most commercial applications, the zeolite-porous matrix composite is provided as a hard aggregate of discrete particles; in the form of extrudates, microspheres, tablets, pellets, granules and the like which substantially retain their shape and strength in use.
Techniques for incorporating zeolites in a matrix are conventional in the art as set forth in U.S. Pat. No. 3,140,253.
U.S. Pat. No. 4,091,007 to Dwyer et al. represents a radical departure in the synthesis of ZSM-5 and it discloses a method for the preparation of ZSM-5 with which the instant invention is concerned. U.S. Pat. No. 4,091,007 relates to a method of preparing ZSM-5 zeolite as a discrete particle having a crystallinity of greater than 40 percent by preforming the reaction mixture into pellets or extrudates which retain their shape and acquire substantial strength during the crystallization process. This reaction mixture contains a source of alkali metal cations and tetralkylammonium cations, silica, alumina and water. The crystallized product can be handled in subsequent chemical processing, such as ion exchange, without necessitating cumbersome processes such as filtration. Further, these discrete particles can be used directly as catalysts after appropriate processing but without the need of any reformulation or pelletizing since the non-crystalline portion of the discrete particle serves as the porous matrix of the prior art compositions.
Another method of preparing in-situ crystallized zeolites of a preformed clay aggregate which are useful in the novel process of this invention is disclosed and claimed in U.S. patent application Ser. No. 591,723, filed Mar. 21, 1984. The disclosure of said application as well as all patents mentioned in the instant specification are herein incorporated by reference.
In said application, an improved method for preparing a crystalline, high silica zeolite is provided which comprises an in-situ synthesis of a high silica zeolite in preformed discrete particles. By employing high silica zeolite seeds in preparing the preformed composite particles, a highly crystalline product is obtained by means of an in-situ crystallization in the absence of the organic compounds required in other high silica zeolite in-situ syntheses. Following the preforming operation, the discrete particles are calcined and then contacted with an alkali metal hydroxide or other hydroxide solution to achieve the desired degree of crystallization. The integrity of the composite particles is retained during the crystallization to provide a zeolite composition in particulate form which is attrition resistant and highly stable.
Said co-pending application relates to an improvement in the process of preparing a crystalline zeolite having a silica to alumina mole ratio greater than 12 and a Constraint Index between about 1 and about 12 by reaction under hydrothermal conditions of a reaction mixture comprising water and sources of silica, alumina and alkali cations, said improvement, to provide discrete particles containing said zeolite, comprises:
(a) mixing seeds of a zeolite having a silica to alumina mole ratio greater than 12 and a Constraint Index between about 1 and about 12, a source of silica, a source of alumina and water to form discrete particles;
(b) thermally treating said particles under conditions effective to provide hard, dry, attrition resistant particles;
(c) mixing said attrition resistant particles with a source of alkali metal hydroxide or other hydroxide to form an aqueous reaction mixture having a composition effective to form the zeolite;
(d) maintaining said aqueous reaction mixture under hydrothermal conditions effective to form said zeolite whereby said zeolite is formed in said discrete particles; and
(e) recovering discrete particles having said zeolite formed therein.